Many aircraft components are made from composite laminate materials. For example, a stringer in an aircraft wing, stiffeners and spar structures, may all be in the form of composite laminate structures. While the structures may be strong when loaded in the way in which they have been designed to be loaded, they may be vulnerable to impact damage, which may result in delamination occurring. Delamination may result in the structure weakening and eventually failing. Particularly vulnerable are the edges of composite laminate structures (at which the layers of the composite laminate structure terminate) where an impact occurs, particularly if the impact is end-on (i.e. when the impact is in a direction perpendicular to the edge and parallel to the plane of the laminates that make up the structure).
If an impact has a significant component of force in the end-on direction the impact may be capable of damaging the bonding between the laminate's layers comprising the composite laminate structure and causing, or adding to the effect of, delamination. Such impacts may be as a result of workmen dropping tools whilst working on the structure, during maintenance for example when the end surfaces of the composite laminate structures are exposed in a way they are usually not. A further way in which the composite laminate structures may be damaged is as a result of wear over time, such wear for example resulting from workmen sitting, or standing, on the structure or contacting the structure directly or by means of tools, or other equipment. It is for example common for a workman to rest tools or toolboxes on the end surface of such structures while working on the aircraft. The edge of a composite laminate structure is typically exposed, at least during maintenance, to direct wear and direct impacts as described above. Similar impact threat exists during the manufacturing and assembly of the composite parts.
When designing an aircraft component, factors that affect the strength and other mechanical properties of composite laminate structures of the aircraft, such as those factors described above, are taken into account. Thus, factors such as those described above typically mean that the composite laminate structures need to be provided with extra strength, resulting in an increase in size and weight.
One known approach to strengthening the edge of a laminar composite is to apply an edge protector—for example as described in US2006/0234007. Another solution is to form the composite laminate with a roll-form edge—that is, an edge formed by a folded layer. However a problem with these conventional methods is that they may not provide sufficient visual evidence of an impact. Such visual evidence may be a requirement of the structure.